Adherence to mucous membranes is an important initial event in bacterial colonization and may be related to their pathogenicity. Pili, shown to be the adherence mechanism in several bacterial species, have been recently demonstrated in H. influenzae. Studies were initiated to isolate and purify hib pili, to evaluate their role in pathogenesis and immunity. Pili expression of several H. influenzae type b strains from disease isolates; CSF and epiglottitis and from carriers, was enriched utilizing their adherence properties to human RBC, as verified by EM. Two of these Hib strains were grown on solid media homoginized and pili isolated by several cycles of isoelectric precipitation followed by ammonium sulfate precipitation. Final purification utilized preparative gel electrophoresis. Rabbit antisera were prepared with these pili preparations.